1. Field of the Invention
This invention relates to non-invasive means of determining cardiac output in patients, and specifically relates to partial re-breathing systems and methods for determining cardiac output in patients.
2. Statement of the Art
It is important in many medical procedures to determine or monitor the cardiac output of a patient. Techniques are known and used in the art which employ the use of catheters inserted at certain arterial points (e.g., femoral artery, jugular vein, etc.) to monitor blood temperature and pressure in order to determine cardiac output of the patient. Although such techniques can produce a reasonably accurate result, the invasive nature of the procedure has high potential for morbidity and mortality consequences.
Adolph Fick""s measurement of cardiac output, first proposed in 1870, has served as the standard by which all other means of determining cardiac output have been evaluated since that date. Fick""s well-known equation, written for CO2, is:   Q  =            V              CO        2                    (                        C                      v                          CO              2                                      -                  C                      a                          CO              2                                          )      
where Q is cardiac output, VCO2 is the amount of CO2 excreted by the lungs and       C          a              CO        2              ⁢      xe2x80x83    ⁢  and  ⁢      xe2x80x83    ⁢      C          v              CO        2            
are the arterial and venous CO2 concentrations, respectively. Notably, the Fick Equation presumes an invasive method (i.e., catheterization) of calculating cardiac output because the arterial and mixed venous blood must be sampled in order to determine arterial and venous CO2 concentrations.
It has previously been shown, however, that non-invasive means may be used for determining cardiac output while still using principles embodied in the Fick Equation. That is, expired CO2 (xe2x80x9cpCO2xe2x80x9d) levels can be monitored to estimate arterial CO2 concentrations and a varied form of the Fick Equation can be applied to evaluate observed changes in pCO2 to estimate cardiac output. One use of the Fick Equation to determine cardiac output in non-invasive procedures requires the comparison of a xe2x80x9cstandardxe2x80x9d ventilation event to a sudden change in ventilation which causes a change in expired CO2 values and a change in excreted volume of CO2. The commonly practiced means of providing a sudden change in effective ventilation is to cause the ventilated patient to re-breath a specified amount of previously exhaled air. This technique has commonly been called xe2x80x9cre-breathing.xe2x80x9d
Prior methods of re-breathing have used the partial pressure of end-tidal CO2 to approximate arterial CO2 while the lungs act as a tonometer to measure venous CO2. That method of re-breathing has not proven to be a satisfactory means of measuring cardiac output because the patient is required to breath directly into and from a closed volume in order to produce the necessary effect. However, it is usually impossible for sedated or unconscious patients to actively participate in inhaling and exhaling into a bag. The work of some researchers demonstrated that the Fick Equation could be further modified to eliminate the need to directly calculate venous PCO2 (PVCO2) by assuming that the PVCO2 does not change within the time period of the perturbation, an assumption that could be made by employing the partial re-breathing method. (See, Capek et al., xe2x80x9cNoninvasive Measurement of Cardiac Output Using Partial CO2 Rebreathingxe2x80x9d, IEEE Transactions On Biomedical Engineering, Vol. 35, No. 9, September 1988, pp. 653-661.)
Known partial re-breathing methods are advantageous over invasive measuring techniques because they 1) are non-invasive, 2) use the accepted Fick principle of calculation, 3) are easily automated, 4) require no patient cooperation and 5) allow cardiac output to be calculated from commonly monitored clinical signals. However, known partial re-breathing methods have significant disadvantages as well. Specifically, known methods 1) are less accurate with non-intubated or spontaneously breathing patients, 2) only allow intermittent measurements (usually about every four minutes), 3) result in an observed slight, but generally clinically insignificant, increase in arterial CO2 levels, and 4) do not permit measurement of shunted blood flow (that is, blood which does not participate in gas exchange). Further, known apparatus used for partial re-breathing techniques are of standard construction and do not compensate for differences in patient size or capacities. In addition, many devices employ expensive elements, such as three-way valves, which render the devices too expensive to be used as disposable units.
Thus, it would be advantageous to provide a means of measuring cardiac output using partial re-breathing techniques which 1) overcome the disadvantages of prior systems, 2) provide better and more continuous measurement, and 3) require less expensive equipment, thereby making the device suitable for manufacturing as a single-use, or disposable, product. It would also be advantageous to provide partial re-breathing apparatus which is instantaneously adjustable to compensate for various sizes and capacities of patients. Further, it would be advantageous to provide new methods of estimating cardiac output based on alveolar CO2 output rather than end-tidal CO2 as is currently used in the art.
In accordance with the present invention, apparatus and methods for measuring cardiac output using a modified Fick Equation are provided where the amount of deadspace which is provided in the apparatus can be adjusted to increase or decrease the volume of exhalate to be re-breathed by the patient, thereby decreasing ventilation without changing airway pressure. The apparatus and methods of the present invention also provide an adjustability factor which enables the apparatus to be adjusted to suit any size or capacity of patient. The apparatus of the present invention also employs significantly less expensive elements of construction, thereby rendering the device disposable.
The apparatus and methods of the present invention apply a modified Fick Equation to calculate changes in pCO2 flow and concentration to evaluate cardiac output. The traditional Fick Equation, written for CO2 is:   Q  =            V              CO        2                    (                        C                      v                          CO              2                                      -                  C                      a                          CO              2                                          )      
where Q is cardiac output (when calculated using re-breathing techniques referred to as pulmonary capillary blood flow or xe2x80x9cPCBFxe2x80x9d), VCO2 is the output of CO2 from the lungs and       C          a              CO        2              ⁢      xe2x80x83    ⁢  and  ⁢      xe2x80x83    ⁢      C          v              CO        2            
are the arterial and venous CO2 concentrations, respectively. It has been shown in prior work of others that cardiac output can be estimated from calculating the change in pCO2, as estimated by end-tidal CO2 (xe2x80x9cetCO2xe2x80x9d), as a result of a sudden change in ventilation. That can be done by applying a differential form of the Fick Equation as follows:   Q  =                    V                  CO                      2            1                                      (                              C                          v              1                                -                      C                          a              1                                      )              =                  V                  CO                      2            2                                      (                              C                          v              2                                -                      C                          a              2                                      )            
where Ca is arterial CO2 concentration, Cv is venous CO2 concentration, and the subscripts 1 and 2 reference measured values before a change in ventilation and measured values during a change in ventilation, respectively. The differential form of the Fick Equation can, therefore, be rewritten as:       Q    =                            V                      CO                          2              1                                      -                  V                      CO                          2              2                                                            (                                    C                              v                1                                      -                          C                              a                1                                              )                -                  (                                    C                              v                2                                      -                          C                              a                2                                              )                      or      Q    =                            Δ          ⁢                      xe2x80x83                    ⁢                      V                          CO              2                                                Δ          ⁢                      xe2x80x83                    ⁢                      C                          a                              CO                2                                                        =                        Δ          ⁢                      xe2x80x83                    ⁢                      V                          CO              2                                                s          ⁢                      xe2x80x83                    ⁢          Δ          ⁢                      xe2x80x83                    ⁢          et          ⁢                      xe2x80x83                    ⁢                      CO            2                              
where xcex94VCO2 is the change in CO2 production in response to the change in ventilation,   Δ  ⁢      xe2x80x83    ⁢      C          a              CO        2            
is the change in arterial CO2 concentration in response to the change in ventilation, xcex94etCO2 is the change in end-tidal CO2 concentration and s is the slope of the CO2 dissociation curve. The foregoing differential equation assumes that there is no appreciable change in venous CO2 concentration during the re-breathing episode, as demonstrated by Capek, et al., in their previous work. Also, a dissociation curve, well-known in the art, is used for determining CO2 concentration based on partial pressure measurements.
In previous partial re-breathing methods, a deadspace, usually comprising an additional 50-250 ml capacity of air passage, was provided in the ventilation circuit to decrease the effective alveolar ventilation. In the present invention, a ventilation apparatus is provided with an adjustable deadspace to provide the necessary change in ventilation for determining accurate changes in CO2 production and end-tidal CO2 commensurate with the requirements of differently sized patients. In one embodiment of the ventilation apparatus, selectively adjustable deadspace is provided through which the patient exhales and inhales. Thus, the adjustable deadspace of the apparatus permits easy adjustment of the deadspace to accommodate any size or capacity of patient, from a small to a large adult. As a result, the patient is provided with a volume of re-breathable gas commensurate with the patient""s size which decreases effective ventilation without changing the airway pressure. Because airway and intra-thoracic pressure are not affected by the re-breathing method of the present invention, cardiac output is not significantly affected by re-breathing. In an alternative method, the deadspace may be effectively lessened by selectively leaking exhaled gas from the ventilation system to atmosphere or to a closed receptacle means during inspiration.
The ventilation apparatus of the present invention includes a tubular portion, which is placed in contact with the patient, and an inhalation conduit and exhalation conduit. In a common configuration, the inhalation conduit and exhalation conduit may be interconnected between a ventilator unit and the patient. Alternatively, however, a ventilator unit (i.e., a source of deliverable gas mechanically operated to assist the patient in breathing) need not be used with the ventilation apparatus and inhaled and exhaled breath is merely taken from or vented to atmosphere. Other conventional equipment commonly used with ventilator units or used in ventilation of a patient may be used with the inventive ventilation apparatus, such as a breathing mask.
An electrical pneumotachometer for measuring flow of gas and a capnograph for measuring CO2 concentrations are provided in proximity to the tubular portion between the inhalation and exhalation portions of the ventilation apparatus and the patient""s lungs. The pneumotachometer and capnograph serve as detection apparatus for detecting changes in gas concentrations and flow and are in electrical communication with a computer having software designed to store and evaluate the measurements taken by the detection apparatus in real time. Other forms of detection apparatus may be used. Adjustable deadspace means are provided in connection with the exhalation portion of the ventilation apparatus, and may interconnect with the inhalation portion of the ventilation apparatus. In one embodiment, the adjustable deadspace means may be manually adjusted. Alternatively, electromechanical means may be interconnected between the computer and the adjustable deadspace means to provide automatic adjustment of the deadspace volume responsive to the size or capacity of the patient and responsive to changes in ventilation.
In an alternative embodiment, a tracheal gas insufflation apparatus is used to provide the change in ventilation necessary to calculate pulmonary CO2 changes using the differential Fick Equation. Tracheal gas insufflation (xe2x80x9cTGIxe2x80x9d) apparatus is commonly used to flush the deadspace of the alveolar spaces of the lungs and to replace the deadspace with fresh gas infused though insufflation means. That is, fresh gas is introduced to the central airway to improve alveolar ventilation and/or to minimize ventilatory pressure requirements. TGI apparatus is interconnected to a ventilator system and includes a means of introducing fresh gas into the breathing tube as it enters the patient""s lungs. The TGI apparatus may be used in the methods of the present invention to determine baseline measurements of VCO2 and etCO2 during TGI. When the TGI system is turned off, a deadspace is formed by the patient""s trachea and the endotracheal tube of the TGI apparatus which allows measurement of a change in CO2 to be evaluated in accordance with the invention. Further, the catheter of the TGI apparatus may be variably positioned within the trachea of the patient to further adjust the deadspace volume.
The deadspace provided in the apparatus of the present invention causes a rapid drop in VCO2 which thereafter increases slightly and slowly as the functional residual lung gas capacity equilibrates with the increase in alveolar CO2 level. The change in etCO2 rises more slowly after the addition of deadspace, depending on alveolar deadspace and cardiac output, but then stabilizes to a new level. A xe2x80x9cstandard,xe2x80x9d or baseline, breathing episode is conducted for a selected period of time immediately preceding the introduction of a deadspace (i.e., re-breathing) and VCO2 and etCO2 values are determined based on measurements made during the xe2x80x9cstandardxe2x80x9d breathing event. Those values are substituted as the values       V          CO              2        1              ⁢      xe2x80x83    ⁢  and  ⁢      xe2x80x83    ⁢      C          a              CO2        1            
in the differential Fick Equation. VCO2 and etCO2 values are also determined from measurements taken approximately thirty seconds following the introduction of a deadspace during partial re-breathing to provide the second values (subscript 2 values) in the differential Fick Equation. The period of time during which partial re-breathing occurs and during which normal breathing occurs may be determined by the individual size and lung capacity of the patient. Additionally, the period of time between a re-breathing episode and a subsequent normal breathing episode may vary between patients, depending on a particular patient""s size and breath capacity. Thus, a thirty second time period for a breathing episode is only an average time and may be greater or lesser.
Cardiac output is determined, in the present invention, by estimating alveolar CO2 concentration rather than basing output on end-tidal CO2 concentration, as is practiced in the prior art. Partial pressure values that are obtained from CO2 measurements are converted to a value for gas content in the blood using the dissociation equation known in the art. Thus, a more accurate cardiac output can be determined. In addition, the accuracy of cardiac output is increased by correcting VCO2 values to account for flow of CO2 into the functional residual capacity of the lungs, defined as the volume of gas left in the lungs at the end of an expired breath. The determination of values based on experiential data is processed by the software program to determine cardiac output.
The ventilation apparatus of the present invention employs inexpensive yet accurate monitoring systems as compared to the systems currently used in the art. The methods of the invention allow automatic adjustability of the apparatus for accommodating patients of different sizes and provide consistent monitoring with modest recovery time. Further, the present apparatus and methods can be used equally with non-responsive, intubated patients as well as non-intubated, responsive patients.